As a material exhibiting shape-memory properties, alloy materials and resin materials have been known in the art. Shape-memory alloys find use in pipe Joints and straightening teeth, whereas shape-memory resins in thermal contraction tubes and laminate materials, fastening pins, and medical equipment such as a plaster cast. Unlike a shape-memory alloy, a shape-memory resin has the following merits. The resin can be processed into a complicated shape, has a high shape-recovery efficiency, light weight, readily colorable, and low cost. Because of these merits, the shape-memory resin is expected to enlarge the application fields.
A shape-memory resin has the following features. It can be deformed at a predetermined temperature and such a desirably deformed shape can be fixed by cooling it to room temperature, and recovered to its original shape by heating it again. The shape-memory resin is characteristically constituted of a frozen phase, which is composed of a physical or chemical bonded site (cross-linking point), and a reversible phase, which is composed of a non-cross-linked portion and flowable at a predetermined temperature or more (the Tg or melting point in the reversible phase).
The mechanism of a shape-memory resin will be further specifically explained. Memorizing a shape, deforming a molded product, and recovering the memorized shape are performed as described in steps 1 to 3 below. The conceptual view of the mechanism is shown in FIG. 1.
1. Processed by Molding
When a shape-memory resin is processed by a predetermined method (heating, melting, and solidification), an initial state (original form) consisting of a frozen phase and a reversible phase (rigid state) (Stage (a) and a partially magnified view (b) of FIG. 1) is memorized.
2. Deformation of Molded Product
A molded product can be arbitrarily deformed at a temperature, at which only the reversible phase but the frozen phase melts, that is, not less than the Tg or melting point in the reversible phase, thereby converting the reversible phase into a soft state (Stage (c) of FIG. 1); followed by applying external force to the molded product while maintaining its state (Stage (d) of FIG. 1). When the deformed product is cooled to Tg or less or the melting point or less, it is fixed in a deformed state with the reversible phase completely solidified (Stage (e) of FIG. 1).
3. Recovery of the Memorized Shape
In the molded product arbitrarily deformed, the deformed state of the shape is maintained by the reversible phase forcibly fixed in the meantime. Therefore, when the temperature of the deformed product reaches a temperature at which the reversible phase alone melts, the resin exhibits elasticity (rubber like properties) and comes to a stable state. In this way, its original shape is recovered (Stage (c) of FIG. 1). The initial state of the molded product shown in Stage (b) of FIG. 1 is brought back by further cooling to not more than Tg or a melting point.
The frozen phase herein is classified into a thermosetting type or a thermoplastic type depending upon the type of cross-linking structure. The thermosetting type and the thermoplastic type are known to have intrinsic advantages and disadvantages.
The frozen phase of the thermosetting type shape-memory resin is composed of a covalently cross-linked structure. The thermosetting type shape-memory resin has the following advantages. The resin is highly effective in preventing fluidization of a resin, excellent in shape-recovering properties and dimensional stability, and recovers the original shape at a high speed. On the other hand, it has the following disadvantages. Because of covalent cross-linking, it cannot be remolded, in other words, non-recyclable.
For example, as a specific example of a conventional thermosetting shape-memory resin, mention may be made of trans-1,4-polyisoprene (Patent Document 1: Japanese Patent Laid-Open No. 62-192440), which is a resin formed by cross-linking trans-1,4-polyisoprene with sulfur or peroxide. The frozen phase of the resin is a cross-linking site and the reversible phase is a crystalline part of trans-1,4-polyisoprene. The resin is excellent in shape-recovering properties; however, due to covalent cross-linking, remolding cannot be made as described above, and thus the recycling efficiency of the resin is poor.
On the other hand, the frozen phase of the thermoplastic type shape-memory resin is composed of a crystalline part, glass-state region of a polymer, entanglement of polymers, or metal crosslink. Since the frozen phase can melt by heating, the resin can be remolded, that is, recyclable. This is an advantage of the thermoplastic type shape-memory resin. However, since the binding force of the frozen phase of the thermoplastic type shape-memory resin is weaker than the thermosetting type having covalent cross-linking. The thermoplastic type is inferior in shape recovering property to the thermosetting resin.
As an example of a conventional thermoplastic type shape-memory resin, mention may be made of polynorbornene (Patent Document 2: Japanese Patent Laid-Open No. 59-53528). The document describes that a polymer-entangled portion serves as a frozen phase, whereas a non-polymer-entangled portion as a reversible phase, providing shape-recovering properties. However, the shape-memory resin of this type has problems. It takes long to recover the shape. Because of a large molecular weight, processability is low.
Polyurethane (Patent Document 3: Japanese Patent Laid-Open No. 2-92914) is also known as an example. The frozen phase is crystalline phase and the reversible phase is amorphous phase. However, the shape-memory resin of this type also takes long to recover the shape. Furthermore, the tensile strength is extremely low, so that it is difficult to use it in component parts for electronic apparatuses.
A styrene-butadiene copolymer (Patent Document 4: Japanese Patent Laid-Open No. 63-179955) is also known. The frozen phase is a glass state region composed of polystyrene and the reversible phase is a crystalline portion composed of trans-polybutadiene. Also in the shape-memory resin of this type, a long shape-recovery time and a low recovery rate are pointed out as problems.
A method for improving shape-memory properties of the thermoplastic type shape-memory resin has been proposed. For example, Patent Document 5 (Japanese Patent No. 2692195) discloses that a shape-memory resin excellent in shape recovery rate and shape recovery time can be provided by hydrogenating not less than 80% of the olefinic unsaturated bond of a ternary compound system block copolymer, which is analogous to the resin of Patent Document 4. However, in Non-Patent Document 1 (Masao Karouji, “Development of Shape-memory Polymers”, CMC, pages 30-43, 1989), the fact that a styrene-butadiene based-thermoplastic shape-memory resin is repeatedly deformed, the recovery rate of a shape-memory resin is reduced, is pointed out as a problem.
Recently, as environmental issue becomes a great matter of concern, the recycling efficiency of a material becomes more important. However, there are no conventional shape-memory resins having not only recycle efficiency but also excellent shape-recovery property, for the reasons set forth above. Therefore, it has been difficult to employ a conventional shape-memory resin to form a molded body requiring recycle efficiency and excellent shape-recovery property, for example, in component parts for electronic apparatuses.
An example in which a thermo-reversible cross-linking structure is introduced into a shape-memory resin, thereby imparting processability and recycle efficiency is shown in Patent Document 6 (Japanese Patent Laid-Open No. 2-258818). The document discloses, as a thermo-reversible covalent cross-linking structure, an ion cross-linking group such as a carboxyl group and a covalent cross-linking structure using the Diels-Alder reaction or a dimerization reaction of a nitroso group. In the claims, a cross-linking body is recited, which is obtained by thermoreversibly cross-linking a base polymer, that is, a block copolymer of an aromatic vinyl monomer and a conjugate diene based monomer, and which is characterized in that the glass-transition temperature (Tg) of the dissociated polymer (the base polymer) of the cross-linked body is higher than the dissociation (cleavage) temperature (Td) of the thermo-reversible crosslink contained in the cross-linked body, and that the glass transition temperature falls within the range of 70° C. to 140° C.
In the document, page 3, lower right column, there is a description reading: “the dissociation temperature of the thermo-reversible crosslink is satisfactory if it is lower than the glass transition temperature of the dissociated polymer, practically, the dissociation temperature is preferably lower by 10° C. or more than the glass transition temperature of the dissociated polymer”. In the block copolymer, an aromatic resin having Tg of about 100° C. serves as a frozen phase as is in the case of the styrene-butadiene copolymer (Patent Document 4). On the other hand, a crystalline diene polymer having a melting point lower than Tg of the aromatic resin serves as a reversible phase. The thermo-reversible covalent cross-linking structure is introduced into a double bond of the diene polymer. When the polymer is heated to Td or more, the bond (crosslink) is dissociated. When the polymer is deformed in this state and then cooled to Td or less, rebonding (re-cross-linking) takes place to provide shape-memory properties. Also, when the polymer is heated to not less than Tg of an aromatic resin, the moldability is improved and the polymer can be remolded. In other words, in the shape-memory resin, the resin portion serves as a frozen phase and the crosslink portion serves as a reversible phase as shown in FIG. 2 (Stage (a)). When the shape-memory resin is heated to Td or more, the crosslink is dissociated (Stage (b)). Furthermore, when external force is applied to the resin while maintaining the heating state, the resin is deformed (Stage (c)). When the resin is cooled to less than Td to permit to form a crosslink again, the shape is memorized as shown in Stage (d). In order to return the original shape, the resin is heated again to Td or more to dissociate the crosslink and then cooling it. In the process of remolding, the resin is heated to not less than the Tg of an aromatic resin. As a result, the resin portion of the frozen phase becomes flowable and thus remolding can be made, as shown in Stage (e).
However, since the crosslink is dissociated in a shape-recovery period, the resin is classified into a thermoplastic type. This means that excellent recovery property cannot be obtained and that types of available resin and thermo-reversible binding crosslink structure are limited. Furthermore, since the dissociation temperature of the covalent cross-linking structure is high (Diels-Alder: 120° C.-160° C., nitroso group: 70° C. to 160° C.), the resin and cross-linking site satisfying the conditions of this patent are actually limited. In particular, the temperature range of a shape-memorizing process is extremely limited. From these, this resin is extremely impractical.
Note that, Non-Patent Document 2 (Engle, et al., J. Macromol. Sci. Re. Macromol. Chem. Phys., Vol. C33, No. 3, pages 239-257, 1993) describes, as examples of the thermo-reversible reaction to be used in cross-linking, the Diels-Alder reaction, nitroso dimerization reaction, esterification reaction, ionene reaction, urethanization reaction, and azlactone-phenol addition reaction.
Non-Patent Document 3 (Yoshinori Nakane and Masahiro Ishitoya, et al., Coloring Material, Vol. 67, No. 12, pages 766-774, 1994), Non-Patent Document 4 (Yoshinori Nakane and Masahiro Ishitoya, et al., Coloring Material, Vol. 69, No. 11, pages 735-742, 1996), and Patent Document 7 (Japanese Patent Laid-Open No. 11-35675) describe a thermo-reversible cross-linking structure using a vinylether group.
An example in which a reversible reaction based on the esterification reaction of an acid anhydride is used to improve thermal resistance and recycle efficiency, is described in a document such as Patent Document 8 (Japanese Patent Laid-Open No. 11-106578). To describe more specifically, in this process, a carboxylic anhydride is introduced in a vinyl polymer compound and cross-linked by a linker having a hydroxyl group.
However, none of the documents include descriptions about shape memory properties and applications as a shape-memory resin.
On the other hand, to deal with environment issue not by recycling but by discarding, shape-memory resins composed of various types of biopolymers have been proposed. For example, Patent Document 9 (Japanese Patent Laid-Open No. 9-221539) discloses a biodegradable shape-memory resin composed of an aliphatic polyester resin such as a polylactic resin. However, these are thermoplastic resins, so that shape recovering properties and recovery rate are insufficient.
A shape-memory resin using a biodegradable thermosetting or photosetting resin is described in Patent Document 10 (National Document of International Patent No. 2002-503524). FIG. 5 of the patent document shows that a shape is memorized by photo-cross-linking and the shape is recovered by dissociation of a crosslink with heat or light. However, no mention is made of recycling of the biodegradable thermosetting or photosetting resin.
Patent Document 1: Japanese Patent Laid-Open No. 62-192440
Patent Document 2: Japanese Patent Laid-Open No. 59-53528
Patent Document 3: Japanese Patent Laid-Open No. 2-92914
Patent Document 4: Japanese Patent Laid-Open No. 63-179955
Patent Document 5: U.S. Pat. No. 2,692,195
Patent Document 6: Japanese Patent Laid-Open No. 2-258818
Patent Document 7: Japanese Patent Laid-Open No. 11-35675
Patent Document 8: Japanese Patent Laid-Open No. 11-106578
Patent Document 9: Japanese Patent Laid-Open No. 9-221539
Patent Document 10: National Document of International Patent No. 2002-503524
Non-Patent Document 1: Masao Karouji, “Development of material for shape-memory polymer”, CMC, pages 30-43, 1989
Non-Patent Document 2: Engle, et al., J. Macromol. Sci. Re. Macromol. Chem. Phys., Vol. C33, No. 3, pages 239-257, 1993
Non-Patent Document 3: Yoshinori Nakane and Masahiro Ishitoya, et al., Coloring Material, Vol. 67, No. 12, pages 766-774, 1994
Non-Patent Document 4: Yoshinori Nakane and Masahiro Ishitoya, et al., Coloring Material, Vol. 69, No. 11, pages 735-742, 1996